题目内容：
根据材料，回答问题。Better Solar Energy Systems: More Heat, More Light
Solar photovoltaic thermal energy systems, or PVTs, generate both heat and electricity, but(51) __________ now they haven't been very good at the heat-generating part compared to a stand-alone solar thermal collector. That's because they operate at low temperatures to cool crystallinesilicon solar cells,(52) __________ lets the silicon generate more electricity but isn't a very efficientway to gather heat.
That's a problem of economics. Good solar hot-water systems can harvest much moreenergy than a solar-electric system at a substantially lower(53) __________. And it's also a spaceproblem: photovoltaic cells can (54) __________ up all the space on the roof, leaving little room forthermal applications.
In a pair of studies, Joshua Pearce, an associate professor of materials science andengineering, has devised a(55) __________in the form of a better PVT made with a different kindof silicon. His research collaborators are Kunal Girotra from Thin Silicon in California andMichael Pathak and Stephen Harrison from Queen's University, Canada.
Most solar panels are made with crystalline silicon, but you can also make solar cells out ofamorphous silicon, commonly known (56)__________thin-film silicon. They don't create as muchelectricity, but they are lighter, flexible, and cheaper. And, because they(57) __________ muchless silicon, they have a greener footprint. (58) __________, thin-film silicon solar cells arevulnerable to some bad news physics in the form of the Staebler-Wronski effect.
"That means that their efficiency drops when you (59) __________ them to light -- pretty much the worst possible effect for a solar cell," Pearce explains, which is one of the(60) __________thin-film solar panels make up only a small fraction of the market.
However, Pearce and his team found a(61) __________ to engineer around the Staebler-Wronski effect by incorporating thin-film silicon in a new(62) __________ of PVT. You don't haveto cool down thin-film silicon to make it work. In fact, Pearce's group discovered that byheating it to solar-thermal operating temperatures, near the boiling(63) __________ of water, theycould make thicker cells that largely(64) __________the Staebler-Wronski effect. When theyapplied the thin-film silicon directly to a solar thermal energy collector, they also found that bybaking the cell once a day, they (65) ___________ the solar cell's electrical efficiency by over 10percent.
请在第__(51)__处填上正确答案。 A．until
B．unless
C．when
D．if
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